Re: [PATCH -v2] mm: Don't use radix tree writeback tags for pages in swap cache

From: Mel Gorman
Date: Thu Sep 01 2016 - 04:52:48 EST


On Wed, Aug 31, 2016 at 02:30:31PM -0700, Andrew Morton wrote:
> On Wed, 31 Aug 2016 10:14:59 +0100 Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
>
> > > 2506952 __ 2% +28.1% 3212076 __ 7% vm-scalability.throughput
> > > 1207402 __ 7% +22.3% 1476578 __ 6% vmstat.swap.so
> > > 10.86 __ 12% -23.4% 8.31 __ 16% perf-profile.cycles-pp._raw_spin_lock_irq.__add_to_swap_cache.add_to_swap_cache.add_to_swap.shrink_page_list
> > > 10.82 __ 13% -33.1% 7.24 __ 14% perf-profile.cycles-pp._raw_spin_lock_irqsave.__remove_mapping.shrink_page_list.shrink_inactive_list.shrink_zone_memcg
> > > 10.36 __ 11% -100.0% 0.00 __ -1% perf-profile.cycles-pp._raw_spin_lock_irqsave.__test_set_page_writeback.bdev_write_page.__swap_writepage.swap_writepage
> > > 10.52 __ 12% -100.0% 0.00 __ -1% perf-profile.cycles-pp._raw_spin_lock_irqsave.test_clear_page_writeback.end_page_writeback.page_endio.pmem_rw_page
> > >
> >
> > I didn't see anything wrong with the patch but it's worth highlighting
> > that this hunk means we are now out of GFP bits.
>
> Well ugh. What are we to do about that?
>

It'll stop silent breakage so

Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Whoever hits it will need to take similar steps we had to with page->flags
by making some 64-bit only, removing flags or inferring the flag values
from other sources.

--
Mel Gorman
SUSE Labs